The present invention relates to flux-encased solder preforms having as the flux casing a flux-binder mixture, wherein the proportion of flux is from 75 to 95% wt. % and the proportion of binder is from 5 to 25 wt. %, and to a process for the preparation of such flux-encased solder preforms.
The joining of metal work pieces, in particular in open-flame soldering, involves the use of fluxes in addition to the solder. Their function is to remove the disruptive oxide films from the base metal before the solder is melted and to protect the bright metal surfaces from further oxidation. Fluxes in the form of powders or pastes are used extensively in industry as soldering additives. The fluxes are generally applied to the work pieces as a paste or suspension, by spraying, painting or dipping, after which they are heated to melting temperature, and the base metals are materially joined to one another by the addition of solder. However, flux-coated preforms and soldering wires are also used in practice, in addition to this method of supplying the flux and solder in separate manner. Such flux-sheathed soldering wires may be used in direct manner for soldering without further additives. When the flame is first applied, the flux first melts and removes the oxide films. Further raising of the temperature results in the solder also becoming molten, bonding the members of the joint in known manner.
The preparation of flux-encased solder preforms has long been known. Typically, plastic flux-binder mixtures are applied to the solder preforms by compression, extrusion or other mechanical processes. Organic polymers, which impart a certain resilience to the flux casing after hardening and thus reduce the risk of shedding under bending stresses, are in many cases used as binders.
Thus, DE-OS 23 36 442 (German published application) describes a flux which contains methacrylate polymers as binders. The latter organic binders, however, suffer from the serious disadvantage that organic decomposition products of the binder, which arise during the soldering operation, generally reduce the efficacy of the fluxes and most commonly impair the flow of the solder. During heating, the organic auxiliary substances evaporate or burn off, resulting in undesirable sooting at the soldering site, and constituting an odor nuisance and health hazard for the operator. Moreover, when organic binders such as methacrylates (DE-OS 25 36 442) or polyisobutylenes (GB 2 027 617) are used in the preparation of the flux-sheathed solder preforms, flammable organic solvents which are injurious to health (for example methyl acetate, trichloroethylene, xylene) are frequently used. The latter solvents are emitted to the surrounding atmosphere during the subsequent drying process.
Those skilled in the art have therefore sought to use mixtures which contain no organic constitutes for such fluxes. Thus, in German patent 24 28 808 a process is described for the preparation of flux-encased solder preforms which contain alkali metal tetraborates as the flux. Such flux mixtures are heated to temperatures from 80 to 250.degree. C., the cooled product is moistened with water, and the plastic composition is applied to the solder preforms, where it hardens. While such flux encasings have the advantage of containing no organic constituents, they also have the disadvantage of being relatively brittle and consequently highly sensitive to mechanical stresses. The result is that at the slightest bending or impact stress whole sections of the flux casing are shed, thus leaving insufficient flux available for the soldering operation.
DE OS 25 29 190 describes an organic-free flux encasing in which the addition to the flux mixture of glass fibers or metal fibers is intended to achieve greater resilience. However, sizeable quantities of glass fibers must be added to the flux to achieve good flexural strengths. The non-homogeneous melting of the flux components, however, results in an increased risk of undesirable changes in the action of the flux. Although the above document also proposes using fibers in the composition of the flux, these can barely be prepared.
Furthermore, the process when producing these flux-sheathed soldering wires is not as secure as when finely ground compressed material is used, because extrusion nozzles very easily become clogged with the longer fibers, thus disrupting the encasing operation.
DE-OS 23 36 442 also addresses the use of soluble silicates (water glass), but these are likewise brittle after hardening.
An object of the present invention is to provide flux-encased solder preforms that can avoid organic constituents that can lead to hazardous organic decomposition products.
Another object of the present invention is to avoid brittleness in flux formulations.
Yet another object of the present invention is to develop flux-encased resilient solder preforms having as the flux casing a flux-binder mixture, wherein the proportion of flux, consisting of mixtures of borates and complex fluorides, should be from 75 to 95 wt. % and the proportion of binder should be from 5 to 25 wt. %, the flux-binder mixture containing no organic substances, and the flux casing when hardened being resilient and being unaffected by mechanical deformation.
A further object of the invention is to develop a process for preparing such flux-encased solder preforms, wherein such a flux-binder mixture is applied in a plastic state to the solder preform.